During cardiac surgery, the natural blood pumping action of the patient's heart and the oxygenation function of the lungs are temporarily replaced by a heart-lung machine which drains blood from the patient, oxygenates it, and pumps it back into the patient's circulatory system. The blood circulation path through the heart-lung machine typically includes a blood reservoir arranged to allow observation of the blood level therein, or to provide some other indication of the amount of blood in the reservoir; a pump such as a roller pump whose pumping action can be accurately controlled; and an oxygenator unit arranged to control the blood temperature and exchange the carbon dioxide in the blood for oxygen.
One of the parameters which has clinical significance and needs to be monitored during surgery is the actual blood volume in the patient's circulatory system. For example, the introduction of anesthetic intravenous (IV) fluids, the natural action of the patient's kidneys, and the loss of blood during surgery all produce significant variations in the volume and composition of the fluid circulating in the patient's circulatory system, which need to be addressed by the perfusionist. In the past, clinical data regarding changes in blood volume during surgery was deduced intuitively from blood pressure measurements and fluid level observations in the reservoir. Blood pressure drop might indicate either a decrease in fluid volume from either blood loss or kidney activity, or an expansion of blood vessels from physiological causes. In the former case, increased IV fluid flow rate or transfusion of blood would be called for; in the latter case, the remedy would be constricting medication. Although observation of the reservoir level could distinguish between the former and the latter causes of blood pressure variations, it could not distinguish in the former cause between blood loss or kidney activity.
In order to correctly maintain the physiological status quo of the patient on a continuous basis, a need therefore existed for a way to continuously monitor the actual blood volume (as opposed to the total fluid volume) in the patient's circulatory system.